You can only change you child's last name with the consent of her mother. The court does not consider another change 'going back' to the first. It considers the change an independent action with all the requirements of the original name change.
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If the mother agrees and truly nothing was done you can might be able to get an order by stipulating to modify to decree or final order. Otherwise, you have to petition and show why it would be in the best interest of the child to now have your last name.
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